Organic light-emitting diode (OLED) devices, also referred to as organic electroluminescent (EL) devices, have numerous well known advantages over other flat-panel display devices currently in the market place. Among these advantages are brightness of light emission, relatively wide viewing angle, and reduced electrical power consumption compared to, for example, liquid crystal displays (LCDs) using backlighting.
Applications of OLED devices include active matrix image displays, passive matrix image displays, and area lighting devices such as, for example, selective desktop lighting. Irrespective of the particular OLED device configuration tailored to these broad fields of applications, all OLEDs function on the same general principles. An organic electroluminescent (EL) medium structure is sandwiched between two electrodes. At least one of the electrodes is light transmissive. These electrodes are commonly referred to as an anode and a cathode in analogy to the terminals of a conventional diode. When an electrical potential is applied between the electrodes so that the anode is connected to the positive terminal of a voltage source and the cathode is connected to the negative terminal, the OLED is said to be forward biased. Positive charge carriers (holes) are injected from the anode into the EL medium structure, and negative charge carriers (electrons) are injected from the cathode. Such charge carrier injection causes current flow from the electrodes through the EL medium structure. Recombination of holes and electrons within a zone of the EL medium structure results in emission of light from this zone that is, appropriately, called the light-emitting zone or interface. The emitted light is directed towards an observer, or towards an object to be illuminated, through the light transmissive electrode. If the light transmissive electrode is between the substrate and the light emissive elements of the OLED device, the device is called a bottom-emitting OLED device. Conversely, if the light transmissive electrode is not between the substrate and the light emissive elements, the device is referred to as a top-emitting OLED device.
The organic EL medium structure can be formed of a stack of sublayers that can include small molecule layers and polymer layers. Such organic layers and sublayers are well known and understood by those skilled in the OLED art.
Because light is emitted through an electrode, it is important that the electrode through which light is emitted be sufficiently light transmissive to avoid absorbing the emitted light. Typical prior art materials used for such electrodes include indium tin oxide and very thin layers of metal. However, the current carrying capacity of electrodes formed from these materials is limited, thereby limiting the amount of light that can be emitted from the organic layers.
The present invention is directed to a method of making a top-emitting OLED device.
In top-emitting OLED devices, light is emitted through an upper electrode or top electrode which has to be sufficiently light transmissive, while the lower electrode(s) or bottom electrode(s) can be made of relatively thick and electrically conductive metal compositions which can be optically opaque.
In conventional integrated circuits, bus connections are provided over the substrate to provide power to circuitry in the integrated circuit. These busses are located directly on the substrate or on layers deposited on the substrate, for example on planarization layers. In complex circuits, multiple levels of bus lines are located over the substrate and separated by insulating layers of material. For example, OLED displays sold by the Eastman Kodak Company utilize multiple bus lines located on the substrate and on various planarization layers. However, these busses are not useful to provide power to the light transmissive upper electrode in an OLED device because conventional photolithography techniques destroy the organic layers and thin upper electrode necessary for a top-emitting OLED device.
U.S. Patent Application Publication 2002/0011783 A1 proposes to solve this problem by the formation of auxiliary electrodes in contact with the upper or top electrode. The auxiliary electrode may be either above or below the upper electrode. The auxiliary electrode has greater thickness and conductivity thereby increasing the current carrying capacity of the upper electrode. However, this approach has difficulties in that it reduces the light emitting area of the OLED device and is difficult to manufacture. In particular, if the auxiliary electrode is formed before the organic elements are deposited, forming a good electrical contact between the upper and auxiliary electrodes is difficult, because the organic materials will be deposited on the auxiliary electrode. Moreover, undesirable moisture can infiltrate through materials at the corners of the auxiliary electrode and the conformal deposition of an additional upper electrode protection and encapsulation layer is problematic. If the auxiliary electrode is deposited above the upper electrode, a patterned deposition process is relatively difficult and liable to destroy both the upper electrode and the organic layers beneath it.
A second prior art method to address this problem is to use an auxiliary electrode, as proposed by U.S. Patent Application Publication 2001/0043046 A1 by Fukunaga et al. entitled “Luminescent Apparatus and Method of Manufacturing the Same.” However, this approach requires a complex multistep processing method and suffers from the above described difficulties.
U.S. Patent Application Publication 2002/0158835 A1 by Kobayashi et al. entitled “Display Device and Method of Manufacturing the Same”, discloses the use of auxiliary wiring elements which are electrically connected to a light transmissive second or upper electrode of an active matrix type planar display device. The auxiliary wiring elements are formed in the same layer or on the same surface as first or lower electrodes, and the auxiliary wiring elements are electrically insulated from the first electrodes. However, Kobayashi et al. provide no drawings describing process steps used in a method of making the device. Moreover, the electrical connection disclosed by Kobayashi et al. is formed between partition walls. The construction of suitable partition walls adds complexity to the process, reduces yields, adds cost, and limits the resolution of the interconnections.
The use of lasers and other techniques to form patterns in integrated circuits is known. For example, U.S. Pat. No. 6,468,819, entitled “Method for Patterning Organic Thin Film Devices Using a Die”, describes the use of a die to form patterns and references the use of laser ablation to form patterns. U.S. Pat. No. 6,444,400, entitled “Method of Making an Electroconductive Pattern on a Support”, likewise describes ablation, including the use of infrared lasers. Other patents, for example U.S. Pat. No. 6,433,355 issued Aug. 13, 2002, entitled “Non-Degenerate Wide Bandgap Semiconductors as Injection Layers and/or Contact Electrodes for Organic Electroluminescent Devices”, also describe the use of laser ablation for patterning. However, none of these methods address problems with power distribution in a top-emitting OLED device.